Scientists have reconstructed the genetic code of a strain of bacteria that caused one of the most deadly pandemics in history nearly 1,500 years ago.

They did it by finding the skeletons of people killed by the plague and extracting DNA from traces of blood inside their teeth.

This plague struck in the year 541, under the reign of the Roman emperor Justinian, so it's usually called the Justinian plague. The emperor actually got sick himself but recovered. He was one of the lucky ones.

"Some of the estimates are that up to 50 million people died," says evolutionary biologist David Wagner at Northern Arizona University. "It's thought that the Justinian plague actually led partially to the downfall of the Roman Empire."

The plague swept through Europe, northern Africa and parts of Asia. Historians say that when it arrived in Constantinople, thousands of bodies piled up in mass graves. People started wearing name tags so they could be identified if they suddenly collapsed.

Given the descriptions, scientists suspected that it was caused by the bacterium Yersinia pestis — the same kind of microbe that later caused Europe's Black Death in the 14th century.

The bacteria get spread by fleas. After someone gets infected from a flea bite, the microbes travel to the nearest lymph node and start multiplying. "And so you get this mass swelling in that lymph node, which is known as a buboe," says Wagner. "That's where the term bubonic plague comes from."

The Justinian plague has been hard to study scientifically. But recently, archaeologists stumbled upon a clue outside Munich.

Housing developers were digging up farmland when they uncovered a burial site with graves that dated as far back as the Justinian plague.

"They found some [graves] that had multiple individuals buried together, which is oftentimes indicative of an infectious disease," Wagner says. "And so in this particular case, we examined material from two different victims. One of those victims was buried together with another adult and a child, so it's presumed that they all may have died of the plague at the same time."

Skeletons were all that was left of the pair. But inside their teeth was dental pulp that still contained traces of blood — and the blood contained the DNA of plague bacteria.

By decoding the bacteria's DNA, Wagner and a team of international scientists could trace the pathogen's evolutionary journey.

They think the strain of bacteria that caused the Justinian plague jumped from rodents into humans and then died out, the team wrote Tuesday in the journal The Lancet Infectious Diseases. The later emergence of Black Death seems to have been caused by a separate event.

The DNA also suggests that, like Black Death, the original source of the plague was in China, says microbiologist Paul Keim, another member of the research team at Northern Arizona University.

"So the ecological reservoir for plague, the historical reservoir, is in China," Keim says. "And it's this emergence, this pattern over and over again, with people moving commodities, rats and fleas around the world that we're able to document."

Overall, this ancient strain is not that different from modern ones that still circulate in places like Arizona, says Keim.

"The biology of the pathogen no doubt could cause another pandemic if it weren't for the changes in human culture and medicine," Keim says.

These days, though, antibiotics can quickly stop plague outbreaks in their tracks.

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Transcript

STEVE INSKEEP, HOST:

When people hear the words bubonic plague, they usually think of the Black Death, the epidemic that hit Europe in the 14th century. You learn about this in school. But the first recorded plague pandemic actually happened 800 years earlier, during the days of the Roman Empire.

NPR's Nell Greenfieldboyce reports that scientists have now recovered the entire genetic code of the bacteria responsible for that ancient pandemic.

NELL GREENFIELDBOYCE, BYLINE: This plague struck in the year 541, when the Roman emperor was named Justinian, so it's usually called the Justinian Plague. He actually got sick himself, but recovered. He was one of the lucky ones.

DAVID WAGNER: Some of the estimates are that up to 50 million people died.

GREENFIELDBOYCE: David Wagner is an expert on microbial genetics at Northern Arizona University. He says the plague swept through Europe, North Africa and parts of Asia.

And it's thought that the Justinian plague actually led partially to the downfall of the Roman Empire.

Contemporary accounts of this plague made scientists suspect it was caused by the bacteria Yersinia pestis, the same kind of microbe that later caused Europe's Black Death. The bacteria gets spread by fleas. After someone gets infected from a flea bite, the bacteria travel to the nearest lymph node and start multiplying.

WAGNER: And so you get this mass swelling in that lymph node, which is known as a buboe. That's where the term bubonic plague comes from.

GREENFIELDBOYCE: The Justinian plague has been hard to study scientifically, since it happened around 1,500 years ago. But recently new housing developments being built outside of Munich led to some old farmland being dug up and people uncovered a burial site with graves that dated as far back as the Justinian Plague.

WAGNER: They found some that had multiple individuals buried together, which is oftentimes indicative of an infectious disease. And so in this particular case we examined material from two different victims. One of those victims was buried together with another adult and a child, so it's presumed that they all may have died of plague at the same time.

GREENFIELDBOYCE: All that was left was skeletons - with teeth. Inside the teeth was the dental pulp that still contained traces of blood. And the blood contains the remains of plague bacteria. Wagner and an international team of collaborators managed to extract enough DNA to reconstruct the ancient pathogen's entire genetic code.

In the journal Lancet Infectious Disease, the team says it looks like this strain of bacteria jumped from rodents into humans and then died out. The later emergence of the Black Death seems to have been caused by a separate event. The DNA also suggests that like the Black Death, the original source of the plague was China.

PAUL KEIM: So the ecological reservoir for plague, the historical reservoir, is in China and it's this emergence, this pattern over and over again, with people moving commodities, rats, and fleas around the world, that we're able to document.

GREENFIELDBOYCE: Paul Keim worked on the study at Northern Arizona University. He says overall this ancient strain is not that different from modern ones that still circulate in places like Arizona.

KEIM: The biology of the pathogen no doubt could cause another pandemic if it weren't for the changes in human culture and medicine.

GREENFIELDBOYCE: He says these days antibiotics quickly stop plague outbreaks in their tracks.